United States Patent [19]
`Chang et al.
`
`llllllllllllllIIIIllllllllll||l|lllllllllllllllllllllllllllllllllllllllllll
`5,577,033
`*Nov. 19, 1996
`
`US005577033A
`[11] Patent Number:
`[45] Date of Patent:
`
`[54]
`
`[75]
`
`LOCAL AREA NETWORK TRANSMISSION
`EMULATOR
`
`Inventors: Eugene Y. Chang, Cambridge; David
`.
`-
`-
`.
`-
`B. Richardson, Bellmgham, Bennett
`C- Baker Dedham, all of Mass’
`’
`
`[73]
`
`.
`_
`.
`Asslgnee- HM?fsmgz’iTfSys‘ems’ Inc" Granada
`’
`'
`
`[*l
`
`_
`_
`N‘mce-
`
`_
`The term °fth1§Pt¥entsha11n°teXt°nd
`beyond the expiration date of Pat. No.
`5,280,481.
`
`[21]
`
`[22]
`
`[63]
`
`[51]
`
`[58]
`
`[56]
`
`APPL NO; 253,753
`
`Filed:
`
`Jun. 3, 1994
`
`Related US, Application Data
`
`.
`Continuation of Ser. No. 97,851, Jul. 27, 1993, Pat. No.
`5,323,388, which is a continuation of Ser. No. 763,340, Sep.
`20’ 1991’ Pat- N°~ 5’289481-
`Int. Cl.6 ........................... .. H04L 12118; H04L 12/28
`US. Cl. ....................... .. 370/60; 370/85.13; 370/94_1
`Field of Search ......................... .. 370/60, 60.1, 85.1,
`370/852, 853, 85,4, 855, 8513, 85_14,
`94.1, 94.2; 340/8255, 825.51; 395120002,
`200.06, 200.10, 200.20
`
`References Cited
`
`US. PATENT DOCUMENTS
`
`‘4,787,082 11/1988 Delaney eta]. ........................ .. 370/85
`Cl al. ..... ..
`43681866 9/1989 wtlhémst “
`4,926,420
`5/1990 Shrmrzu .... ..
`.
`.
`370/94.1
`5/1990 Shnmzu .... ..
`4,930,123
`370/60 1
`5 093 827 3,1992 Franklin cl al
`. . . 5370/8513
`5:280:481
`1/1994 Chang et a]. '. . . . . . .
`5,323,388
`6/1994 Chang et a1. ........................... .. 370/60
`
`''' " 380/49
`370/94.1
`
`FOREIGN PATENT DOCUMENTS
`
`OTHER PUBLICATIONS
`
`IBM Technical Disclosure Bulletin, Short Hold Mode in
`ISDN’ v01‘ 32’ No‘ 10A’ Mar‘ 1990’ pp‘ 47-49‘
`IEEE Std 802~1990 IEEE Standards for Local and Metro
`_
`_
`_
`politan Area Networks: Overview and Architecture (excerpt)
`(Dec. 31, 1990).
`US Government Open Systems Interconnection Pro?le
`(oosrp), Version 2.0, Oct. 1990.
`,
`Haugdahl, J. Scott, Architecture Technology Corp., Inside
`NETBIOS, 2nd Ed. (1988).
`Leifer, Dory, Draft Internet RFC, University of Michigan
`(Sep_ 1991)‘
`Digital ISDN Connectivity, LAN Bridges and Gateways
`(1990).
`Well?eet Communications, Inc. Router/Bridge PRI, LANs,
`Bridges, Gateways and Routers (1991).
`Network Express, Inc. ISDN PRI Bridging, LAN s, Bridges,
`Gateways and Routers (1991).
`The Software Link Networking with PC-MOS, ISDN LAN
`(1990).
`Interactive Systems ISDN LAN Bridging, LAN Bridgesand
`Gateways (199())_
`-
`PROMPTUS T-l Ctnntnseivei+ Flexible Bridge/Router +
`Multiplexer (Feb. 1990).
`EXCELLTECH INC. ISDN and ExcellNET (1989).
`Lanworks ISDN Router for NeWWare (Mar- 1991)
`Intercomputer Communications Corporation (ICC) RIN
`Product overview ($91» 30, 1991)
`.
`.
`(List contlnued on next page.)
`
`Primary Examiner—Melvin Marcelo
`Attorney, Agent, or Firm-Lappin & Kusmer LLP
`
`ABSTRACT
`[57]
`A method and apparatus for using Switched telecommuni
`cations services to emulate a local area network (LAN)
`-
`-
`medium. The method and apparatus convert a public
`.
`.
`.
`.
`swltched network or an equivalent pnvate network rnto a
`LAN cabling methOd for connecting distant devices using
`the Same Communications Software as used in traditiolflally
`wired LANs.
`
`0302646 2/1989 European Pat. OlT. ...... .. H04L 11/16
`
`20 Claims, 4 Drawing Sheets
`
`r
`FISH/70
`01/?! Ht?
`MIA MAI/S
`
` 2007
`
` Ex. 2007-0001
`
`IPR Licensing, Inc.
`Exhibit .
`ZTE Corp v. IPR Licensing, Inc.
`IPR2014-00525 
`
`

`
`5,577,033
`Page 2
`
`OTHER PUBLICATIONS
`
`Teleos Solidi?es PC Terminal Adapter Leadership: 1-7
`(Nov. 26, 1990).
`IBM Product Announcement; [BM ISDN Interface Co—Pro
`cessor/2 Model 2: 1-3 (Jan., 1991).
`3Com NETBuilder Bridge/Router (1991).
`ICL/ISDN Upgrade Package for Personal Computers (Apr.
`1988).
`0ST ISDN Communications for Personal Computer PC
`SNET CARD Product Overview (Aug 10, 1990).
`Fujitsu ISDN INSIGHTS; Fujitsu Demonstrates ISDN LAN
`to LAN and WAN to LAN Connectivity (Fall of 1989).
`DigiBoard Product Pro?le; DigiBoard ISDN Netlink (Jan.
`1991).
`National Institute of Standards and Technology, North
`American ISDN Users Forum, Application Software Inter
`face Expert Working Group, Proposed Standard (Sep.,
`1991).
`
`Northern Telecom, ISDN PC Card N etBIOS Interface (Apr,
`1988)
`IBM, ISDN PC Card, ISDNBIOS Programmers Guide
`(1991).
`AT&T ISDN PC Card Programming Interface (1989).
`Hayes Microcomputer Products, Inc. ISDN PC Card, ISD
`NBIOS Interface (Apr, 1991).
`ICL, ISDN PC Card Application Program Interface (Apr.
`1988)
`0ST, PC SNET Card Programmers Manual, 1991.
`
`Teleos Communications, Applications Interface User’s
`Guide (1990).
`DigiBoard ISDN Netlink, ISDN NetBIOS LAN (1990).
`DigiBoard, NetLINK Technical Reference (Nov., 1990).
`
` Ex. 2007-0002
`
`

`
`U.S. Patent
`
`Nov. 19, 1996
`
`Sheet 1 of 4
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`5,577,033
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`U.S. Patent
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`Nov. 19, 1996
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`U.S. Patent
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`Nov. 19, 1996
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`U.S. Patent
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`Nov. 19, 1996
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`5,577,033
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`

`
`5,577,033
`
`1
`LOCAL AREA NETWORK TRANSMISSION
`EMULATOR
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This is a continuation of U.S. patent application Ser. No.
`08/097,851, ?led Jul. 27, 1993, now U.S. Pat. No. 5,323,
`388, which is a continuation of U.S. patent application Ser.
`No. 07/763,340, ?led Sep. 20, 1991, now U.S. Pat. No.
`5,280,481.
`
`10
`
`FIELD OF THE INVENTION
`
`This invention relates to a method and apparatus for using
`switched telecommunications services to emulate a local
`area network (LAN) medium.
`
`BACKGROUND
`
`Local Area Networks (LAN s) provide a method for
`connecting computers or other devices together to exchange
`data or to harness groups of computers together to apply
`their combined power to a single problem. Generally speak
`ing, a LAN includes: 1) a high speed transmission medium,
`typically metallic or ?ber optic, for connecting each of the
`devices to the LAN; 2) the ability to transmit a message on
`the transmission medium directed to a single device; and 3)
`a means known as “broadcast” in which all devices con
`nected to the LAN medium can receive a message trans
`mitted on the medium. A standard for the implementation of
`LAN devices and systems has been established by the
`Institute of Electrical and Electronic Engineers as IEEE
`Standard 802.
`The physical length of the transmission medium and the
`total number of devices connected thereto are typically
`limited on a LAN due to the physics of high speed trans
`mission systems. Bridges and routers are devices used to
`connect multiple LAN s to provide communications between
`individual LANs and to construct large networks that tran
`scend the technical size limits of a single individual LAN.
`When the individual LANs to be interconnected are at
`geographically remote locations, bridges and routers are
`used in pairs, one at each site, to provide a path for data to
`?ow from one LAN to another, with a lower speed com
`munication link between the bridge or router pair. Typically
`the data rates of the long distance communications link is a
`fraction of the data rate of the LAN medium. The use of
`bridges and routers has been limited, however, due to the
`cost of these devices and the costs of the long distance
`communications link.
`An all-digital telephone network, known as the Integrated
`Services Digital Network (“ISDN”), has become a potential
`substitute for the private long distance lines currently used
`by bridges and routers. ISDN provides relatively high speed
`digital transmission service on an “as needed” basis, and is
`different from LAN transmission media in that it is a
`switched transmission media which provides a point-to
`point transmission service on an intermittent basis.
`Modem communications technology can be analyzed
`with respect to the Open Systems Interconnect (OSI) Ref
`erence Model. The 081 model decomposes a communica
`tion system into seven major components or layers which
`are de?ned by international standards. The 051 model is
`concerned with the interconnection between systems, i.e.,
`the way they exchange information, and not with the internal
`functions that are performed by a given system. The CS1
`
`25
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`35
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`50
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`55
`
`60
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`
`2
`model depicted in FIG. 1 provides a generalized view of a
`layered architecture, using an approach where sets of func
`tions have been allocated to different layers.
`The ?rst layer is known as the physical layer and is
`responsible for the transmission of bit streams across a
`particular physical transmission medium. This layer
`involves a connection between two machines that allows
`electrical signals to be exchanged between them.
`The second layer is the data link layer, and is responsible
`for providing reliable data transmission from one node to
`another and for shielding higher layers from any concerns
`about the physical transmission medium. It is concerned
`with the error-free transmission of frames of data.
`The third layer, the network layer, is concerned with
`routing data from one network node to another and is
`responsible for establishing, maintaining, and terminating
`the network connection between two users and for transfer
`ring data along that connection. There can be only one
`network connection between two given users, although there
`can be many possible routes from which to choose when the
`particular connection is established.
`The fourth layer is the transport layer, and is responsible
`for providing data transfer between two users at an agreed on
`level of quality. When a connection is established between
`two users, the transport layer is responsible for selecting a
`particular class of service to be used, for monitoring trans~
`missions to ensure the appropriate service quality is main
`tained, and for notifying the users if it is not.
`The ?fth layer is the session layer, and it focuses on
`providing services used to organize and synchronize the
`dialog that takes place between users and to manage the data
`exchange. A primary concern of the session layer is con
`trolling when users can send and receive, based on whether
`they can send and receive concurrently or alternately.
`The sixth layer is the presentation layer, and is responsible
`for the presentation of information in a way that is mean
`ingful to network users. This may include character code
`translation, data conversion or data compression and expan
`sron.
`The seventh layer is the application layer, and it provides
`a means for application processes to access the system
`interconnection facilities in order to exchange information.
`This includes services used to establish and terminate the
`connections between users and to monitor and manage the
`systems being interconnected and the various resources they
`employ.
`Different components (or implementations) that conform
`to a common standard are considered equivalent and inter
`changeable. A system constructed from components that
`conform to their respective standard is expected to interop
`erate (i.e., to be able to communicate) with any other system
`constructed out of a different set of components that conform
`to the standards. Communications between systems are
`organized into information that is exchanged between enti
`ties at each layer.
`A layer in the 051 model provides speci?c services to an
`upper layer through service access points (“SAPs”). Take,
`for example, the situation where Systems A and B are joined
`by a transmission medium at layer 1. Information from layer
`x of system A is constrained to communicate with layer x of
`system B. The information of layer x of system A is
`transported, however, by requesting service from layer x-l
`of system A for delivery to layer x of system B. The
`mechanism for communication between two systems at a
`single layer is referred to as a protocol (i.e., “a layer x
`protocol”), and a protocol stack is a set of protocols for
`
` Ex. 2007-0007
`
`

`
`5,577,033
`
`15
`
`20
`
`3
`layers 1 to x. The 051 protocols provide ?exibility in usage
`by incorporating optional features and user determined
`parameters. Pro?les are standards that specify the selection
`of options and parameters to ensure compatibility between
`two compliant systems. Pro?les are needed since two com
`pliant systems using diiferent pro?les may still not be able
`to exchange data.
`In FIG. 1, layer 1 represents the network or transmission
`medium, and includes token rings, token buses, and inter
`faces such as RS-232, RS-530 and v.35. Layer 2, the data
`link layer, has as its primary responsibility the transfer of
`frames of information between physically linked devices.
`When only two devices are connected by the network layer
`medium, the data link layer assumes that the network layer
`will provide the mechanism of addressing messages to the
`proper device.
`IEEE Standard 802.2 provides a model which divides the
`data link layer 2 into two sublayers: an upper sublayer for
`Logical Link Control (LLC) and a lower sublayer for Media
`Access Control (MAC). The IEEE 802.2 model differs from
`earlier data link layers of the OSI Reference Model by
`providing a method for addressing messages to speci?c
`destinations. This is required since more than two devices
`are connected by the medium at layer 1. This mechanism is
`25
`necessary in the context of a single isolated LAN (or LAN
`segment) without connections to other LAN s (or LAN
`segments) because many devices are connected to a com
`mon transmission medium and a means for directing a
`message to a single destination is important.
`The MAC sublayer regulates station access to the trans
`mission medium that is shared by multiple stations on the
`LAN. For a given LAN, the MAC sublayer governs a
`common transrrrission medium that has one pathway or route
`between communicating network stations. In the context of
`the IEEE 802.2 model, the network station address is
`referred to as the MAC address and is su?icient for ensuring
`delivery of a MAC frame to a destination address on the
`LAN. The MAC sublayer offers services consistent with
`those in the 051 data link layer.
`The LLC sublayer mediates multiple logical connections
`for upper layer service users. As a service provider, the LLC
`sublayer offers several Service Access Points (SAP) as
`logical ports for multiple upper layer entities located at a
`given network station address. As a service user, the LLC
`sublayer issues requests through the SAP provided by the
`MAC sublayer. The LLC sublayer Service Access Points are
`typically shown situated between layer 3 (network) and
`layer 2 (data link) of the 051 Reference Model.
`A signi?cant number of layer 3 protocols bypass the LLC
`50
`Service Access Point and interface directly to the MAC
`Service Access Point.
`
`30
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`35
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`45
`
`4
`FIG. 1 shows the relationship of the invention with
`respect to the 081 Reference Model. The generally accepted
`role of the ISDN in the communications industry or 081
`Reference Model is shown as a stack occurring in layers 1
`to 3 of FIG. 1 (bottom left). The ISDN has the role of a layer
`3 service with service access points to layer 4 protocols. Use
`of the disclosed invention permits ISDN to be used as an
`alternative LAN medium, thus permitting existing computer
`systems and other communication devices designed to use
`LAN 5 to be connected through the ISDN without change of
`protocols from layer 3 on up. This allows access to the ISDN
`for a large body of systems and software without requiring
`modi?cation.
`The disclosed embodiment of the invention features a
`MAC layer interface, packet replication to emulate broad
`casting on a common access medium, physical connection
`during periods with message traf?c, physical disconnection
`during periods with no message tra?ic, classi?cation of
`traf?c patterns with re-direction to circuit and packet
`switched channels that match the required capacity, a virtual
`channel interface that utilizes multiple physical channels to
`service one logical channel, a virtual physical interface that
`makes multiple physical interfaces appear as a single physi
`cal interface, and a method for providing connections to a
`number of users that exceed the number of physical chan
`nels.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`The present invention will be better understood herein
`after as a result of the detailed description of the invention
`when taken in conjunction with the following drawings in
`which:
`FIG. 1 diagrammatically depicts the 081 Reference
`Model and its relation to the invention;
`FIG. 2 is a block diagram of an embodiment of the
`invention for conveying a directed datagram;
`FIG. 3 is a block diagram of an embodiment of the
`invention for conveying a broadcast datagram; and
`FIG. 4 is a block diagram of an embodiment of the
`invention for conveying a multicast datagram.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Three classes of datagrams are typically submitted to a
`LAN medium: directed datagrams, multicast datagrams and
`all stations broadcast datagrams. The LAN emulator man
`ages LAN datagram traf?c by a set of logical channels
`between every pair of nodes that exchanges datagrams. The
`actual transmission of datagrams between nodes is provided
`by a physical channel. The LAN emulator only requires a
`physical channel between nodes when datagrams are
`actively being exchanged over a logical channel (between
`two nodes).
`Since the hardware interface to the public network (e.g.
`ISDN) provides a limited number of physical channels, the
`LAN emulator provides a monolithic interface to the higher
`layer protocol process. That process interacts with a single
`entity, the LAN emulator, while the total transmission ser~
`vice may be provided by more than one hardware interface
`to the public network.
`All types of datagrams (directed, multicast, and broad
`cast) intended for transmission are potentially subject to one
`or more ?ltering mechanisms. A ?lter can either leave the
`datagram unchanged or remove the datagram from any
`
`SUMMARY OF THE INVENTION
`
`The disclosed invention provides a method and apparatus
`for using the IEEE Standard 802 LLC or MAC service layer
`as an interface to communicate over the ISDN. The dis
`closed invention presents the ISDN as a LAN transmission
`media to upper layer (layer 3 and above) protocols, and
`permits communication systems designed to operate over
`LAN to operate over the ISDN. As a result, LAN devices can
`be dispersed geographically using inexpensive ISDN com
`munications without the geographic limitations of a single
`LAN and without the cost of bridges, routers, and the
`associated communications links currently used to intercon
`nect LAN segments.
`
`55
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`60
`
`65
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` Ex. 2007-0008
`
`

`
`5,577,033
`
`5
`further transmit processing. Datagrarns that remain
`unchanged are termed ordinary or un?ltered datagrams,
`while datagrams that are removed by a ?lter are termed
`?ltered datagrams. Each ?lter typically acts on a speci?c
`class or type of datagram.
`One generic ?ltering mechanism used in the invention is
`termed rate suppression. Rate suppression acts on certain
`types of datagrams which contain repetitive information,
`and functions by passing only a certain percentage or ratio
`of those datagrams it recognizes. The purpose of rate sup
`pression ?ltering is to 9 minimize transmission charges for
`those datagrams whose content does not change or changes
`very slowly over time.
`Another generic ?ltering mechanism used in the invention
`is termed response spoo?ng. Response spoo?ng acts on
`those packets which contain repetitive information, but
`which require a response from the destination or destina
`tions. The response spoo?ng ?lter not only removes these
`datagrams from further transmission processing, but also
`simulates the response that would be expected from the
`destination(s), and delivers the spoofed response to the
`higher layer protocol processes.
`With reference to FIG. 2, an embodiment of the invention
`for conveying a directed datagram with a speci?c destination
`address through the LAN Emulator interface to emulate the
`transmission of a directed datagram on a LAN is described.
`As mentioned above, a protocol stack is a set of protocols
`for the various layers. With tile disclosed embodiment, and
`with reference to FIG. 2, a high layer protocol process
`submits a directed datagram to the LAN Emulator interface
`10 accompanied by the appropriate LAN MAC layer source
`and destination addresses. For every device there is at least
`one LAN MAC address and a corresponding switched
`network address.
`This disclosed embodiment describes a direct interface
`from higher layers to the MAC interface, as is-commonly
`found in systems implemented for personal computers. An
`LLC interface may be required in some systems. The
`difference between an LLC interface and a MAC interface is
`not signi?cant with respect to the disclosed invention.
`A directed datagram ?lter 15 makes a determination of the
`datagram type by comparing the datagram type with the
`contents of a directed datagram ?lter list 25. This is a list of
`protocol speci?c datagrams speci?c to the application sys
`tem. When there is a matching datagram type in the directed
`datagram ?lter list 25, the datagram is marked as one which
`may be discarded from the transmission queue or enqueued
`for a spoofed response at a later time. The marking is based
`upon the actions speci?ed within the directed datagram ?lter
`list 25.
`The datagram is then passed to a channel classi?er 20 for
`further quali?cation. However, datagrams marked for
`removal from the transmission queue are not given to the
`charmel classi?er 20, and so are not sent to any destination
`node. Instead, they are either discarded or sent to the
`datagram response handler 30, where a spoofed response is
`formatted and eventually delivered to the higher layer pro
`tocol process.
`The channel classi?er 20 receives un?ltered directed
`datagrams from the directed datagram ?lter 15. Using the
`LAN MAC address as a search key, the classi?er 20
`retrieves a switched network address and an associated node
`charmel status from a network de?nition table 35.
`The network de?nition table 35 has an entry for each node
`on the emulated LAN. A node may have multiple entries
`with di?erent LAN MAC addresses. Each entry includes,
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`20
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`25
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`30
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`35
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`50
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`55
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`65
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`6
`but is not limited to: the LAN MAC address, which is the
`node LAN address the device would have if it was connected
`to a conventional LAN; the switched network address,
`which is the address of the device on the switched network
`(e. g., for devices using ISDN, the public network number on
`the public switched telephone network); the node type,
`which is a descriptor which describes the node type or
`function; suspension timer values, which are the parameters
`that control the suspension of the network connection;
`preferred service parameters, which specify the preferred
`types of transmission service when a connection is created;
`node channel status parameter, which re?ects the operational
`status of a remote node; and broadcast service selectors,
`which are parameters that specify the methods by which
`Broadcast messages are distributed to remote nodes.
`One class of preference is the type of service. The
`effective bandwidth delivered is determined by the preferred
`service parameter of the network de?nition table. The chan
`nel classi?er can decide whether to transmit low priority
`information on low bandwidth channels, such as the D~chan
`nel packet switched service of ISDN. Devices connected to
`an ISDN may select, among others, a B-channel circuit
`switched service, B-channel packet switched service,
`D-channel packet switched service, or H0 circuit switched
`service.
`Another class of preference is the minimum and maxi
`mum throughput desired. With the disclosed invention,
`multiple instances of a physical interface may be used under
`a single service layer. The upper layer protocols can there
`fore be presented with a single logical service layer while the
`actual transmission service may be delivered by more than
`one physical interface. It is possible to synthesize higher
`speed transmission service by combining multiple physical
`interfaces under a single service interface. Thus a device that
`requires higher speed transmission may specify the mini
`mum and maximum number of transmission channels to be
`used when communication is established with a remote
`node. For purposes of description of the invention, the
`logical channel is the connection service delivered to the
`upper layers, and the physical channel is the means by which
`datagrams are delivered.
`The datagram is discarded by channel classi?er 20 when
`there is no entry in the table 35 that has the destination LAN
`MAC address. For LAN MAC addresses that are in the
`network de?nition table 35, the four possible values for node
`channel status are: registered with a logical channel and
`assigned physical channel(s), registered with a logical chan
`nel and no physical channels, registered, or not registered.
`Where the node channel status for a case is registered
`node with datagram tra?ic on both a logical channel and its
`physical channels, it is considered a connection that is
`completely active. An un?ltered datagram is immediately
`submitted to the datagram dispatcher 40.
`For an un?ltered datagram where the node channel status
`is registered node with datagram traf?c on the logical
`channel but with no associated. physical channel, there is an
`attempt to establish a connection to the destination node on
`a new physical channel. This is accomplished by giving the
`datagram to channel manager 45 which attempts to establish
`a connection on a physical channel and then to send the
`datagram over that connection. The channel manager 45
`manages the process of establishing a connection through
`both a logical and new physical channel.
`Node registration is a MAC management function that
`occurs at the time the LAN Emulator is initialized. When a
`node is registered with another node it means that it will
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`5,577,033
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`20
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`25
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`7
`respond to a request for connection. It also means that
`another node may attempt a connection with it. There is no
`implication that the connection attempt will be successful; a
`connection attempt may fail because the node’s circuit
`resources may be occupied at the time of the attempt.
`Node de-registration occurs when the LAN Emulator is
`shut down. This involves the LAN emulator sending de
`registration messages to its connection partners. De-regis
`tration is not mandatory. A node may also be de-registered
`when an attempt to connect to it fails because that node is no
`longer active.
`> Datagrams addressed to a node which is not registered are
`discarded by the channel classi?er 20.
`As mentioned above, the channel manager 45 is respon
`sible for initiating a connection on a logical and/or a physical
`channel. If there is a request for connection on a pre-existing
`logical channel, the channel manager 45 establishes connec
`tions on the requisite physical channel(s). For a connection
`request when no logical channel exists, the channel manager
`45 will set up a new logical channel as well as new physical
`channels.
`A logical channel may be supported by more than one
`connection to the same destination through multiple physical
`channels. The channel manager 45 uses a user preference
`contained within the network de?nition table 35 to deter
`mine the number of physical channel resources to allocate
`for a given connection attempt to the destination node.
`There are three possible occurrences when there is an
`attempt to allocate a physical channel on behalf of the
`logical channel. In the ?rst, there are either no physical
`channels available for conveying the datagram or there is a
`network problem that prevents a call from being offered to
`the destination node. In the second, there are physical
`channels available, but when a call request is placed to the
`remote node at the destination switched network address, a
`rejected call response gets returned. In these two cases, the
`logical channel is optionally torn down and network de?
`nition table maintenance is performed.
`The third possibility is that the call request is accepted by
`the node at the destination switched network address, at
`which point the connection is considered physically active.
`When the node channel status ?eld shows that no logical
`or physical channel exists, there is a choice for an appro
`priate course of action which is dependent upon whether a
`connection on a physical channel is possible, and which
`requires a destination node to have registered its LAN MAC
`address and to be able to accept a connection request. In this
`case, a logical channel and logical channel reference number
`are assigned after the physical channel is set up and a
`connection established.
`The channel manager 45 will also monitor traffic on the
`physical channel, and during periods when there is no traffic,
`it D may disconnect the physical channel while maintaining
`the logical channel. When new datagram traf?c begins, the
`channel manager 45 will reassign a physical channel. During
`this process a physical channel is described as being sus
`pended and later resumed. The channel manager 45 will
`monitor external events such as incoming calls and deter
`mine whether a logical channel will release its physical
`channel in order to reassign it to the new call. This may be
`termed release of bandwidth on demand.
`One consequence of the invention’s‘ embodiment of sus
`pend and resume is that the channel manager 45 may
`maintain more logical channels than the maximum number
`of physical channels possible. This is known as channel
`over-subscription. The channel manager 45 may associate an
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`8
`idle physical channel to a new logical channel while still
`maintaining the logical channel that had originally used the
`physical channel. As a consequence of over-subscription,
`there may be periods when there is more demand for active
`channels than ther